Sealing compositions for bonding ceramics to metals

ABSTRACT

1,173,386. Ceramic seals. WESTINGHOUSE ELECTRIC CORP. 26 April, 1967 [30 June, 1966], No. 19145/67. Heading B3V. [Also in Division C1]  A method of sealing ceramic or refractory metal members to ceramic members comprises applying a sealing composition to the members, securing the sealing areas together, and heating the assembly to secure a bond, the composition of the sealing material after the sealing operation being, by weight per cent: and 0.5-10% of at least one of: SiO 2  BaO, ZrO 2 , SrO, TiO 2 , BeO, ThO 2 , Y 2 O 3 . Also included may be 0À5-3À5% by weight of Nd 2 O 3 , Ta 2 O 5 , W0 3 , Nb 2 O 5 , V 2 O 5 . A chart is included listing examples. The method may be applied to sealing Ta, Nb or ceramic, e.g. alumina, discs 12; 14 or caps on the ends of high-temperature metal vapour lamps 10 of alumina. Alumina rings 24, 26 are sealed to the discs 12, 14. One method preparing the sealing composition includes mixing 1 mole of CaCO 2  with ¢ a mole of alumina in powder form and adding the selected other constituents. Amyl acetate and cellulose acetate are included to obtain the desired viscosity and adhesion, and the composition is applied by brushing or spraying, after sand-blasting and cleaning the metal. The seals are air-dried before the assembly is located in a molybdenum clamping fixture, Fig. 2, having a tantalum R.F. susceptor 40. Alternatively, the sealing composition may be formed into compacted preforms. The fixture is placed on a rod. 48 within the R.F. coil of a vacuum furnace and subjected to a heating schedule, Fig. 3 (not shown). Another method of preparing the sealing composition to avoid a degassing period in the furnace includes melting the composition in a gas crucible furnace, pouring the molton mixture into water, ball-milling the mixture in distilled water to a fine powder, adding amyl acetate and cellulose acetate and subjecting the mixture to ultrasonic vibrations. A different firing schedule, Fig. 4 (not shown), may then be used. After evacuation of the tube 10 and insertion of the discharge sustaining filling, the tantalum tube 18 is closed and welded at 22. The expansion ratio of the tube 10, end plates and the sealing material match each other.

Sept. 30; 1969 GREK|LA ET AL 3,469,729

SEALING COMPOSITIONS FOR BONDING CERAMICS TO METALS Filed June 30, 1966a l I r /4O V2000 .H, l I600 u I. i200 0 I0 3040 5 so 7080 90100 JTIME-MINUTES n M M U Q I [U 05 a 5 I g :"g a 32 -32 5 0 I0 2 '40 so soe0 TIME-MINUTES INVENTORS Richard B. Grekil0,Shih-Ming H0, William J.Knochel and Francis C'M. Lin

BY 544 KM ATTORNEY United States Patent 3,469,729 SEALING COMPOSITIONSFOR BONDING CERAMICS TO METALS Richard B. Grekila and Shih-Ming Ho,Pittsburgh, Pa., William J. Knochel, West Orange, N.J., and Francis C.M. Lin, New York, N.Y., assignors to Westinghouse Electric Corporation,Pittsburgh, Pa., a corporation of Pennsylvania Filed June 30, 1966, Ser.No. 562,016 Int. Cl. H01k 1/42; C04b 37/00, 39/00 US. Cl. 220-23 10Claims This invention relates to scaling compositions for sea]- ingceramics to ceramics and ceramics to metals and more particularly to asealing composition and method of producing same for sealing ceramic orrefractory metal closure members to the ceramic discharge tube of hightemperature metal vapor lamps.

Numerous compositions have been employed in an effort to seal ceramicbodies, particularly ceramic bodies of high alumina content, torefractory metals. Principally these eflorts have been directed to thesealing of either ceramic or metal end discs or caps to ceramic lampbodies composed of sintered high density polycrystalline alumina inconcentrations of the order of 99.5%. These sealing compositions, inaddition to providing a good bond, must also withstand the hightemperatures and alkali metal vapors employed in the discharge of suchlamps. Attempts have been made to seal such lamps utilizing alloys oftitanium-nickel composition, as a metallic bond, and glassycompositions, containing for example aluminum oxide and one or more ofcalcium oxide, barium oxide and strontium oxide at nearly eutecticpercentages. Metallic bonds do not, however, prove suitable for highertemperature stresses in the range of 1500 C., while the glassy sealingcompositions tend to recrystallize at higher temperatures producingcracks, and additionally, reactions between the alkali metal vapors ofthe lamps and the glassy sealing material cause, in some instances,early failure of the lamps.

The foregoing difficulties with respect to known methods aresubstantially eliminated to a large extent in lamps sealed withcompositions envisioned by the present invention. Basically, the raw orbatch compositions of the present invention consist essentially of purecalcium carbonate and pure alumina both in very fine powder form and inmolar proportions of 2: 1, in addition to selective additives whichessentially act as modifying agents in producing consistent hightemperature, vacuum tight bonds or seals between polycrystalline aluminalamp bodies and ceramic or metallic end closure members. These endclosure members are generally high alumina content ceramics orrefractory metals such as tantalum or niobium. Others of the additivestend to improve the thermal properties of the near eutectic calciumoxide-alumina compositions as well as the mechanical strength of thebonds. Of additional importance in producing sound vacuum tight sealswith the compositions of the present invention are the firingtempreatures and time periods employed during the process of sealinghigh temperature ceramic discharge tubes. The sealing processes involvegenerally the heating of a discharge tube structure having the sealingcompositions applied thereto in a vacuum furnace from room temperatureat a predetermined rate until a temperature of between about 1425 C. and1550 C. is reached, holding the assembly at that temperature forapproximately one minute, then reducing the temperature at apredetermined rate to about 700 C. and then permitting the assembly tocool, with the furnace shut down, to room temperature.

An object of this invention is to provide a sealing composition toeffectively seal ceramic members to ceramic 3,469,729 Patented Sept. 30,1969 ICC members or to refractory metal members in a high temperaturemetal vapor lamp.

Another object is to provide a sealing composition which willeffectively retain a vacuum tight bond between ceramic material andmetallic material at temperatures up to 1200 C.

A further object of the present invention is to provide a sealingtechnique for producing a seal which is both thermally and mechanicallysound at extremely high temperatures.

These and other objects as well as many of the attendant advantages ofthe present invention will become more clearly understood as thefollowing detailed description is considered in connection with theaccompanying drawings, in which:

FIGURE 1 is a sectional view of a high temperature metal vapor dischargetube illustrative of one possible use of the sealing compositions of thepresent invention;

FIG. 2 illustrates a fixture for use in sealing ceramic body members tometal end plates in a vacuum furnace;

FIG. 3 is a firing schedule diagram illustrative of one method ofproducing ceramic to metal seals in accordance with the presentinvention; and

FIG. 4 is a firing schedule diagram of another method of producingceramic to metal seals in accordance with the present invention.

One of the principal uses for which the sealing compositions of thepresent invention have been highly successful is the sealing of enddiscs or caps to high temperature metal vapor discharge tubes of thetype shown in FIG. 1. The discharge tube illustrated in FIG. 1 generallycomprises a body member 10 of high-density, sintered, polycrystallinealumina which has sealed thereto and is sealed off at each end bytantalum or niobium discs or caps 12 and 14 preferably of a thicknessfrom about 0.004 to 0.015 inch. Of course these end discs or caps may beequally as well constructed from a ceramic material such as, forexample, a ceramic having a high alumina content. Each of the discs orcaps 12 and 14 have secured to the inner surfaces coiled tungstenelectrodes 16 and on the exterior surfaces lead-in conductor members 18and 20 respectively. Lead-in conductor 18 is in the form of a tantalumtube which extends through end disc 12 to provide for the evacuation ofthe interior of the discharge tube and the insertion into the lamp ofthe discharge sustaining filling. After evacuation and charging of thedischarge tube the tubular lead-in conductor 18 is tipped off by meansof squeezing and welding at 22. Short polycrystalline alumina backuprings 24 and 26 are also sealed to the outer surfaces of tantalum orniobium end discs 12 and 14 respectively. When cup-shaped caps areemployed instead of the end discs illustrated, the alumina backup ringsare' unnecessary.

When the tube of FIG. 1 is sealed off, the mating surfaces betweendischarge tube 10, end discs 12 and 14, and backup rings 24 and 26 arecoated with a pastelike form of the compositions of the presentinvention and held together during the novel firing schedules shown inFIGS. 3 and 4 by means of a molybdenum fixture of the type shown in FIG.2.

In order to obtain a vacuum tight seal, the sealing material must wetboth the ceramic body and the metal disc or cap. At the same time, thethermal expansion of the three different materials (i.e., the ceramicdischarge tube body, the metallic end disc or end cap and the sealingcomposition) should closely match each other to avoid failure as aresult of thermal stresses. Furthermore, the seal must be chemicallyinert to metal vapors and have suflicient bond strength.

As an example, the batch composition from which the various alternativeembodiments are derived consists essentially of one mole of pure calciumcarbonate and /2 mole of pure alumina, both in very fine powder form. Tothis batch mixture is added a selected minor percentage of one or moreof the modifying oxides later to be described. The constituents are wellmixed and then suspended in an organic vehicle, for example amylacetate. Several drops of a viscosity imparting agent such as an organiccement, for example, cellulose acetate, are added in order to increasethe green strength of the composition and the mixture is stirred until apaste-like consistency is reached. The batch composition is then placedon the tantalum tube 18 at its point of contact with the center hole ina tantalum or niobium disc 12 and on the mating areas of backup ring 26,disc 14, are tube 10, disc 12 and backup ring 24 and the assembly firedas will later be described to produce a high temperature vacuum sealbetween the associated parts. As is well known to those skilled in theart, the composition may be applied to the respective surfaces by meansof brushing, spraying or by use of compacted preforms.

Sealing compositions of improved mechanical bonding and better fluxingaction are produced by modifying a basic batch mixture of CaCO and A1 ina percentage by weight ratio of approxvimately 2:1 by adding from /2 toby weight of one or more materials taken from the group consisting ofSiOBaO, ZrO SrO, TiO ThO and BeO. The bonding and fluxing additive in finepowder form is mixed with 90 to 99 /2% by weight of a basic 1:2 molarmixture of A1 0 and CaCO and the resulting composition well mixed. Anorganic vehicle, for example amyl acetate, is added to the mixture toform a thin paste and a few drops of a viscosity imparting agent, forexample cellulose acetate, is added to obtain the desirable viscosityand dry adhesion. The metal parts to be joined are lightly sandblastedwith carborundum grit and then cleaned. The mixture in paste form isthen applied by brushing onto the sealing areas of the parts and whilethe composition in paste form is still wet the parts are joined togetherand held until air dried before being placed in a specially designedmolybdenum clamping fixture to be fired.

The clamping fixture, as shown in FIG. 2, generally comprises a basesupport member having extending therefrom fixture support rods 32. Thebase support mem-- ber 30 additionally carries a centering plug 34, aceramic spacer 36 is slipped over one end of the centering plug 34 andthe joined together lamp placed upon the ceramic spacer. At the otherend a metal plug 37 is placed on backup ring 24 to equalize the heat anda second ceramic spacer 38 placed upon the metal plug 37. A tantalum RFsusceptor 40 is placed around the assembly and a pair of holding clamps42 are placed onto support rods 32 to hold the susceptor 40 in place. Afixture head 44 is placed over the support rods and thrust againstceramic spacer 38. A pair of holding nuts 46 for the assembly are thenthreaded onto the upper ends of support rods 32 to secure the assembly.The base support of the fixture is then placed on a support rod 48within an RF coil and the sealing compositions, now in dry form on themating surfaces of the assembly, are heated in accordance with a definedfiring schedule.

A firing schedule for the batch compositions, above described, is shownin FIG. 3. After the assembled discharge tube is placed in the fixtureand the fixture placed in a vacuum furnace on support rod 48, theassembly is quickly heated from room temperature to approximately 700 C.in about three minutes. The furnace is then held at an averagetemperature of about 800 C. for twenty minutes during a degassingperiod. The furnace temperature is then increased from approximately 900C. to from about 1450 to 1550 C. at a rate of about 40 C. per minute fora period of about 15 minutes. The assembly is held at a temperature frombetween 1450 and 1550 C. for about one minute and then cooled at a rateof about 30 C. per minute for twenty minutes. At this point with thetemperature of the assembly at about 700 C. the furnace is shut down andthe assembly permitted to cool to room temperature. Discharge tubeshaving a inch outside diameter polycrystalline alumina body and filledwith a standard mercury, sodium and argon mixture have been operated formore than 200 hours at 900 watts, or a loading of 50 watts per squarecentimeter, without damage to the seals. In addition, sodium filledlamps with polycrystalline alumina bodies have been operated for morethan 3000 hours at 400 watts without evidence of any damage to sealswhen made with the above described additive compositions and fired inaccordance with the above schedule.

To avoid the necessity for a long degassing period shown in the FIG. 3schedule, an alternative mixing and firing schedule involves thefritting of the composition prior to scaling. After the blending of thebatch composition for a prolonged period, the well mixed composition isplaced in a recrystallized alumina crucible and melted in a gas cruciblefurnace at 1600 to 1650 C. until the mixture becomes molten. This moltenglass is then fritted by pouring immediately into water, then ground andball milled for from 75 to hours in distilled water until the particlesize of the glass is reduced to less than 400 mesh. A size of less than200 mesh, however, is considered sufficient for purposes of producing asatisfactory sealing composition. The amyl acetate and cellulose acetateis then added as previously described and ultrasonic vibrations are usedto disperse the coagulated particles and to insure a finely divided anduniformly mixed paste. The assembled lamp is then placed in the holdingfixture and then placed in a vacuum furnace and ilr rled in accordancewith the firing schedule shown in The firing schedule shown in FIG. 4requires that the assembled lamp be heated from room temperature toabout 700 C. in about 3 minutes as before, then from about 700 C. tobetween 1425 and 1550 C. at a rate of approximately 40 C. per minute forabout 20 minutes. The assembly is then held at a temperature of from1425 to 1550 C. for a period of approximately one minute. After thehold, the assembly is cooled from the holding temperature at a rate ofabout 30 per minute down to 700 C. at which time the furnace is turnedoff and the assembly permitted to cool to room temperature.

Several alternative compositions have been found to provide excellentsealing of polycrystalline alumina tubing to end discs or caps of eitherceramic, such as alumina, or metal when prepared and fired in accordancewith the above-described methods. These compositions further improve themechanical bonding strength and the vacuum sealing quality of the metalto alumina seals. One group of these compositions include the mixing ofa batch composition comprising 1 mole of CaCO and A: mole of A1 0 withthe addition of /2 to 10% by weight of Si0 and from about /2 to 3% byweight of one or more of the materials consisting of Ta O W0 Nb O and NdO Another group of compositions performing equally well consist of thebasic composition of A1 0 and CaCO with the addition of from /2 to 10%of Y O This yttrium oxide composition can be further modified to improvethe vacuum sealing quality and mechanical bonding strength of thecomposition by the addition to the composition of from about /2 to 3% ofone or more of the materials Nd O Ta O W0 Nb O and V 0 Thesecompositions can again be formed and fired in accordance with either ofthe foregoing sealing methods.

The following chart represents 28 specific examples indicated representpercent by weight of the final sealing of the ultimate constituents ofvarious sealing compositions after sealing has been accomplished inaccordance with one of the foregoing methods. The percentagescomposition.

Sealing Composition Percent Sealing Composition Percent 0.5 3.0 2.0 3.0.-L Total, percent N0'1E.-Ba0, ZrO-a, SrO, TiO or YaOs in any of theabove designated compositions.

Basically then, the sealing compositions of the present invention intheir final form must include from between about 44% to 55% by weight ofCaO, from between about 40% to 50% by weight of A1 0 and from betweenabout 0.5 and 10% by weight of one or more materials taken from themodifying group comprising SiO Y O BaO, ZrO SrO, TiO BeO and ThOAdditionaly, in most instances, the composition will also include frombetween about 0.5 and 3.5% by weight, with respect to the total weightof the CaO, A1 0 and the material from said modifying group, one or morematerials taken from another group consisting of Nd O T3205, W03, Nb O5,and V205.

Each of the various sealing composition embodiments provide for the hightemperature sealing of alumina to alumina or alumina to niobium ortantalum metal in a high temperature gas discharge lamp, The sealingcompositions provide good mechanical bonding strength and evidence nodeleterious effects during prolonged operation from the metal vaporsgenerally employed in this type discharge lamp. Furthermore, each of thevarious compositions have a temperature coefiicient of expansion whichmatches to a substantial degree that of the niobium or tantalum metaland the polycrystalline alumina body. As will be further apparent fromthe foregoing, the novel compositions each include the basicconstituents A1 0 and CaCO at nearly eutectic proportions in their batchform in addition to one or more oxide additives and employ either thefrit or powdered compositions fired in accordance with their respectivefiring schedules to provide heretofore unrealized strength anduniformity in high temperature metal vapor lamp seals.

Since numerous changes may be made in the above described embodiments ofthe invention and other embodiments may be made without departing fromthe spirit thereof, it is intended that all matter contained in theforegoing description or shown in the accompanying drawings, shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A sealing composition for bonding ceramics to ceramics or metals,which composition principally composition principally comprises fromabout 44 to 55% by weight of CaO, from about 40 to 50% by weight of A1 0and from about 0.5 to 10% by weight of at least one material selectedfrom the modifying group consisting of SiO,, BaO, ZrO SrO, Ti0 BeO, Th0and Y O 2. A sealing composition according to claim 1, wherein there isadditionally included from about 0.5% to 3.5% by weight, with respect tothe total weight of said CaO,

Bet) and T1102 may be substituted in whole or in part for SiOa A1 0 andthe material from said modifying group, of at least one materialselected from another group consisting Of Ndgog, Tago5, W03, Nb205, andV205.

3. A sealing composition according to claim 1 wherein the material ofsaid modifying group consists essentially of about 3% by weight ofSiO;,.

4. A sealing composition according to claim 2 wherein the material ofsaid modifying group consists essentially of about 2.5% by weight of SiOand the material of said other group is present in total amount of about0.5% by weight.

5. A sealing composition according to claim 2 wherein said material ofsaid modifying group consists essentially of about 2.5 by weight of Y Oand the material of said other group is present in a total amount ofabout 0.5% by weight.

6. A sealing composition according to claim 2 wherein said material ofsaid modifying group consists essentially of about 3l% by weight of Y Oand said materials of said other group are present in a total amount ofabout 3% by weight.

7. The method of sealing niobium or tantalum metal members to aluminaceramic members comprising the steps of:

(a) admixing a predetermined amount of A1 0 CaCO and one or more oxidestaken from the group consisting of SiO;,, BaO, Zr-0 SrO, TiO BeO, T andY O in a finely divided form,

(b) adding liquid vehicle and viscosity-imparting agent to the admixtureto form a paste,

(0) applying the paste so formed on the sealing areas of the metal andceramic members,

(d) joining the sealing areas of the metal and ceramic members andholding together until dried,

(e) placing the held-together metal and ceramic assembly in a vacuumfurnace, and

(f) heat treating the assembly according to a schedule which comprises:

(1) heating from room temperature to approxi mately 700 C.,

(2) degassing at an average temperature of about 800 C. for about twentyminutes,

(3) increasing the temperature from about 900 C. to from between 1450 to1550 at a rate of about 40 C. per minute for a period of about fifteenminutes,

(4) holding the assembly at a temperature of from about 1450 to 1550 forabout one minute, then (5) cooling at a rate of about 30 C. per minutefor about twenty minutes, and

(6) permitting the assembly to further cool until room temperature isreached.

8. The method of sealing an alumina ceramic member to a niobium ortantalum metal member comprising the steps of:

(a) admixing a predetermined amount of A1 CaCO and one or more oxidestaken from the group consisting of SiO BaO, ZrO SrO, TiO BeO, Th0; and YO (b) melting the admixture in a furnace until it becomes molten,

(c) pouring the molten mass immediately into water,

(d) grinding and milling the resultant frit until the particle size isreduced to less than 200 mesh,

(e) adding liquid vehicle and viscosity imparting agent to form a paste,

(f) applying the paste to the sealing ends of the alumina ceramic memberand niobium or tantalum member,

(g) securing the sealing areas of the members together,

and

(h) heating in a vacuum furnace in accordance with a heating schedulewhich comprises:

(1) heating the assembly from room temperature to approximately 700 C.in about three minutes, then heating from about 700 C. to from between1425 to 1550 C. at the rate of approximately 40 C. per minute for abouttwenty minutes,

(2) holding the assembly at a temperature of from between 1425 and 1550C. for one minute,

(3) cooling the the assembly from between 1425 and 1550 C. at a rate ofabout 30 per minute to approximately 700 C., and

(4) permitting the assembly to cool from about 700 C. to roomtemperature.

9. A sealedv discharge device comprising:

(a) a hollow elongated ceramic envelope,

(b) refractory metal or alumina closure members positioned proximate toeach end portion of said ceramic envelope, and

(c) a sealing composition hermetically bonding said closure members tosaid ceramic envelope, said sealing composition principally comprisingfrom about 44 to 55% by weight CaO, from about to by weight A1 0 andfrom about 0.5% to 10% by UNITED STATES PATENTS 2,466,508 4/ 1949 Sun10647 2,805,166 9/1957 Loffier 106-47 2,876,120 3/1959 Machlan 106473,243,635 3/1966 Louden et al -59 XR 3,275,358 9/1966 Shonebarger ,161--196 XR 3,275,359 9/1966 Graff 16l- 196 XR 3,281,309 10/1966 Ross "161-196 3,281,931 11/1966 Ritz 29473.1 3,385,463 5/ 1968 Lange 2202.3

EARL M. BERGERT, Primary Examiner H. F. EPSTEIN, Assistant Examiner U.S.C1.X.R.

9. A SEALED DISCHARGED DEVICE COMPRISING: (A) A HOLLOW ELONGATED CERAMICENVELOPE, (B) REFRACTORY METAL OR ALUMINA CLOSURE MEMBERS POSITIONEDPROXIMATE TO EACH END PORTION OF SAID CERAMIC ENVELOPE, AND (C) ASEALING COMPOSITION HERMETICALLY BONDING SAID CLOSURE MEMBERS TO SAIDCERAMIC ENVELOPE, SAID SEALING COMPOSITION PRINCIPALLY COMPRISING FROMABOUT 44 TO 55% BY WEIGHT CAO, FROM ABOUT 40 TO 50% BY WEIGHT AL2O3, ANDFROM ABOUT 0.5% TO 10% BY WEIGHT OF AT LEAST ONE MATERIAL SELECTED FROMTHE MODIFYING GROUP CONSISTING OF BAO, SIO2, ZRO2, SRO, TIO2, BEO, THO2AND Y2O3.